In the conventional production of blown plastic forms or articles, an extruded tube (parison) of thermoplastic material which is at a temperature above its melting point, is enclosed in an appropriately-shaped mold and pressurized air is used to expand the parison against the wall of the mold to assume the contour of the mold.
Following completion of the blowing cycle, the temperature of the molded article is decreased by cooling to a level below the melting point of the thermoplastic material at which the formed plastic article retains its shape. The length of time required to effect the cooling often takes up a large proportion of the total time required to complete the molding cycle.
The heat is removed from the blown molded article by a variety of procedures, including through the cooled mold walls and through the use of blowing air as a heat transfer medium. To improve the effectiveness of the heat removal portion of the cycle, refrigerants, usually liquid nitrogen, have been introduced into the plastic form, after it has been blown to the shape of the mold.
The conventional manner of introduction of the refrigerant into the formed article is by injection of a refrigerated gas or liquid refrigerant through the blow pin which is used to introduce the blowing air during the blowing step or through a small tube contained within the blow pin.
The cooling medium either is introduced admixed with air or is separately injected in an undirected manner into the blown article. In all blown articles, there are regions which are thicker than others and hence require more cooling time than the thinner portions. The thicker portions thereby become the limiting factor controlling cooling time.
It has been previously suggested in U.S. Pat. No. 3,488,801 to achieve a more uniform cooling of the blow molded article by employing a needle having an angled end which enables cooling fluid to be directed towards a certain locus of the hot thermoplastic material. The needle may be moved during the cooling operation to direct the cooling fluid towards a plurality of different loci on the interior of the parison by rotating and/or longitudinally moving the needle. In this prior art operation, therefore, while differential cooling may be achieved by directing coolant at specific locations, it is possible to apply additional coolant to only one location at one time.